Emulsion polymers prepared from vinyl acetate and ethylene provide wide application as binders in industry. Unfortunately, these binders experience unacceptable loss in strength in the presence of water and other solvents. In addition, they exhibit deficiencies in adhesion to the substrates on which they are used. These shortcomings have been reduced by the use of adhesion promoting or crosslinking comonomers and/or post-added crosslinkers.
The most successful of the many chemistries employed is aminoplast technology, especially N-methylolacrylamide (NMA) and urea-formaldehyde (U/F) condensates. While they are low in cost, compatible with aqueous emulsions, rapidly cured under acid catalysis and substrate reactive, they suffer from two deficiencies: the emission of low levels of formaldehyde, a suspect carcinogen; and inadequate adhesion to certain substrates, for example, metal, glass and synthetics such as mylar.
Several monomers have recently been developed which will overcome these deficiencies, but they exhibit a third deficiency when used in a nonwoven binder. The resulting binder emulsion polymer causes blocking, i.e. difficulty in separating two adjacent sheets of bonded nonwoven material. While polymers containing NMA show minor blocking, which is required for efficient rolling of towels and creping of the web, the new monomers demonstrate very high blocking to the extent that it is extremely difficult to separate adjacent sheets. Exemplary of such new monomers are N-(meth)acrylamidoglycolic acid and a compound of the formula EQU R--NH--(CH.sub.2).sub.n --CH(OR.sup.1).sub.2
wherein R is a C.sub.3 -C.sub.10 olefinically unsaturated organic radical having functionality which renders the nitrogen atom electron deficient, R.sup.1 is hydrogen or a C.sub.1 -C.sub.4 alkyl group and n is 3 or 4.
U.S. Pat. No. 4,289,676 discloses binder copolymers containing at least 85 wt%
(a) a mixture of from 40-60 parts by weight of styrene and/or acrylonitrile and from 60-40 parts by weight of butadiene or
(b) esters of acrylic acid and/or methacrylic acid with alkanols of 1-8 carbon atoms, and/or vinyl esters of acetic acid or propionic acid and/or vinyl chloride, and optionally up to 40 wt% based on total monomers (b), of acrylonitrile, styrene or butadiene, from 0-5 wt% of alpha,beta-monoolefinically unsaturated monocarboxylic acids and/or dicarboxylic acids of 3-5 carbon atoms and/or their amides and 3-10 wt% N-acrylamidoglycolic acid and/or N-methacrylamidoglycolic acid.
U.S. Pat. No. 4,449,978 discloses nonwoven products bonded with a binder comprising a polymer of vinyl acetate/ethylene/N-methylolacrylamide/acrylamide. These nonwoven products have a low residual free formaldehyde content.
U.S. Pat. No. 4,448,908 discloses a latex, the particles of which comprise a polymer core and a shell thereover, the shell comprising a water insoluble monomer of the formula ##STR1##
U.S. Pat. No. 4,647,611, discloses a process for preparing a nonwoven binder emulsion containing a crosslinkable vinyl acetate or vinyl acetate/ethylene copolymer prepared by polymerizing in an aqueous dispersion vinyl acetate or vinyl acetate and ethylene with a crosslinkable comonomer of the formula EQU R--NH--(CH.sub.2).sub.n --CH(OR.sup.1).sub.2
by the "trail" addition of the crosslinkable comonomer.